To the present date, inexpensive and manufacturable patterning of magnetic media in the submicron size scale has been difficult to attain. The limitations of conventional lithographic patterning for dimensions below 0.1 micron (100 nm.) are well known, and are described in "Lithography for ULSI", by S. Okazaki, in a review paper (p. 18, vol. 2440, Proceedings of SPIE). Optical lithography with a light source in the deep ultra-violet ("DUV") is expected to serve in circuit and media fabrication for feature sizes no smaller than about 0.05 micron (50 nm). At present, there are no inexpensive methods for lateral patterning/texturing of solid substrates on a 5 to 50 nm scale.
Also previously, it was difficult or impossible to assemble ordered laminar structures or periodic arrays of particles or objects that are very small (5-20 nm, or 0.005-0.02 micron), and to reliably form such a laminar structure by a method that is simple and inexpensive. In addition, methods to adjust or tailor the lattice spacing in the size range 5-100 nm (0.005-0.1 micron) generally do not exist. There are numerous useful applications of such laminar structures. These include high density magnetic recording media, phased array radiation emitters, radiation sensor arrays, and patterns of electrical contacts/connections for high density interconnections between components. Such patterned electrical contacts are useful in the assembly of stacked integrated circuits.
An ideal method to make such laminar structures described above would have the following features:
1. The method is based on well known procedures and applies to patterning over useful areas (1 to 1,000 cm.sup.2 ).
2. The method allows the spacing between nm scale particles or groupings of such particles to be easily adjusted.
3. The method scales up readily from the laboratory to a manufacturable process.
4. The size distribution, as measured by standard deviation, of the nm scale particles may reach about 20%, rather than the narrower size distribution (e.g. 10%) required by other patterning methods.
It is therefore an object of the present invention to provide such laminar structures as well as methods of fabrication which incorporate all of these features.
Accordingly, this invention proposes the use of a lattice layer, which is made of synthetic deoxyribonucleic acid (DNA), and is designed and fabricated by standard synthetic techniques, and forms by self-assembly of appropriately designed DNA segments. Self-assembly of the DNA lattice is performed in water solution, optionally at an air-liquid interface. The assembled lattice is then transferred onto a substrate surface where it is stabilized. This lattice provides lattice cells or sites which can hold one or more of nm-scale particles, which are thereby assembled into an ordered laminar structure that may have many useful applications.
Also disclosed herein is a chemical affinity/blocking method of assembly. In this method, the substrate surface and the particles are both coated with selected molecules which attract the particles to the substrate surface, and which also enable formation of covalent chemical links between the particles and the surface. The DNA lattice acts to "block" the attractive force between particles and the surface, leaving available lattice cells as attractive sites for particle binding and covalent linking only at the open regions in the DNA lattice layer.
A preferred embodiment provides an organized magnetic recording or storage medium with each bit consisting of about four suitable magnetic particles, and with a well controlled spacing between bits of 25 nm. Such a magnetic storage medium may have an real information density of about 10.sup.12 bit/in.sup.2 (1 terabit or Tbit/in.sup.2.). Each bit occupies about 625 nm.sup.2, and consists of about 4 magnetic particles (optionally crystalline) having a diameter of 8-10 nm. The particles may be ferromagnetic particles comprising a metal such as cobalt, iron, manganese, or nickel. A preferred composition is to alloy one or more of these metals with platinum, palladium or samarium. Alternatively, the particles are made of a ferromagnetic oxide, two examples being BaFe.sub.12 O.sub.19 and SrFe.sub.12 O.sub.19. Optionally, each magnetic particle bit is covered with a thin layer of a noble metal (silver, gold, platinum or palladium). A fabrication method for such a magnetic storage medium is also disclosed.
Moreover, a second embodiment having 9 magnetic particles/bit provides an information density of about 5.times.10.sup.11 bit in.sup.2 ( 0.5 Tbit in.sup.2), and an area/bit of about 1,400 nm.sup.2. Other embodiments with about 16 and 25 particles/bit are also described.
An alternative embodiment of the present invention permits an array of electrical connections between two different parts with each connection made by a metal particle (gold, for example) of diameter 10-50 nm with spacing between said metal particle connections on the scale of 10-50 nm. The metal particle connections are typically arranged in a square lattice pattern.